Magnet alloys

ABSTRACT

A ternary permanent magnet alloy which is composed of 69.5 to 73.0 percent by weight of Mn, 26.4 to 29.5 percent by weight of Al and 0.6 to (1/3 Mn-22.16) percent by weight of C, and primarily consists of a body-centered tetragonal structure phase, and which has magnetic characteristics of BHmax &gt; OR = 1.0 X 106 G.Oe, Br &gt; OR = 2,800 G. and BHc &gt; OR = 1,400 Oe., excellent magnetic stability, mechanical properties, weather resistance and corrosion resistance, and are not susceptible to spontaneous disintegration in the atmosphere.

United States Patent [151 3,661,567

Yamamoto [4 1 May 9, 1972 [54] MAGNET ALLOYS 3,116,181 12/1963 Hokkeling ..148/31.57 3,194,654 7/1965 Kaneko ..148/31.57

[72] inventor: Hiroshi Yamamoto, Osaka, Japan [73] Assignee: Matsushita Electrlc Industrial Co., Ltd., 'f i Rutledge 05am" Japan Assistant E.\'ammer-G. K. White [22] Filed: Dec. 6, 1967 21 App1.No.: 688,502 [57] ABSTRACT Related U.S. Application Data Continuation-impart of Ser. No. 429,260, Feb. 1, 1965, abandoned.

References Cited UNITED STATES PATENTS 7/1957 Morgan ..75/l34 A ternary permanent magnet alloy which'is composed of 69.5 to 73.0 percent by weight of Mn, 26.4 to 29.5 percent by weight of A1 and 0.6 to (1/3 Mn-22. 16) percent by weight of C, and primarily consists of a body-centered tetragonal struc ture phase, and which has magnetic characteristics of BHmax 21.0X10 G.Oe, Br 2800 G. and B H c 1 4QQOe. excellent magnetic sta511it, meEharfiEa1 properties, vie athei r e sistance and corrosion resistance, and are not susceptible to spontaneous disintegration in the atmosphere.

1 Claims, 6 Drawing Figures PATENTEDMM 9 I972 3. 661 ,56 7

SHEET 1 OF 3 INVENTOR uneas ypnmmro ATTORNEYS PATENTEDMAY 9 I972 3, 6 61 ,5 6 7 snmanrs mmvm NW A NM/ m $4; W60? @AW/ W A myv FIG. 4 %42 29 28 27 26 AMWWWZ INV ENT OR Harm/w 7R NRHDTD ATTORNEY MAGNET ALLOYS CROSS-REFERENCE TO RELATED APPLICATION The present application is a Continuation-in-Part application of Ser. No. 429,260 filed Feb. 1, 1965 now abandoned.

BACKGROUND OF INVENTION The present invention relates to manganese-aluminum-carbon (Mn-Al-O) ternary alloys which have excellent magnetic characteristics, corrosion resistance, stability and mechanical properties to be used as a permanent magnet.

Conventional Mn-Al binary alloys have a potential utility as a magnet material but their properties are not sufficiently satisfactory for practical use as a magnet material. For instance, an alloy composed of about 72 percent by weight of Mn and about 28 percent by weight Al, when transformed from the hexagonal crystal structure of e-phase, which emerges at elevated temperatures, into the tetragonal crystal structure of metastable phase (-y-phase), by subjecting it to a suitable heat treatment, exhibits ferromagnetism'and tentatively shows the properties suitable for use as a permanent magnet. However, the BHmax value of the alloy is not greater than about 0.6 X G.Oe. and, in addition, because of its magnetic phase being a metastable phase, the alloy is poor in stability as well as in mechanical properties, so that it is entirely unserviceable for practical applications. There have also been known Mn-Al-C ternary alloys of body-centered tetragonal system containing 2.0 to 5.5 percent by weight of carbon and 0.4 to 15.5 percent by weight of aluminum, but these alloys have magnetic characteristics which are only as good as to be used as a substitute for nickel which does not shows the properties required for permanent magnet at all, and thus are not adapted for use as a material for permanent magnet.

SUMMARY OF INVENTION The object of the present invention, therefore, is to provide a novel permanent magnet material which has excellent magnetic characteristics, stability, mechanical properties and corrosion resistance, and which primarily consists of a Mn-AI-C ternary alloy, by incorporating into the conventional Mn-Al binary alloy carbon as the third element, to thereby convert.

said binary alloy into a ternary alloy within a specific composition range so as to improve the magnetic characteristics of the binary alloy drastically.

More specifically, the present invention relates to a magnet material which is composed of 69.5 to 73.0 percent by weight of Mn, 26.4 to 29.5 percent by weight of Al and not less than 0.6 percent by weight but not more than 166 Mn-22.16 percent by weight of C, and which consists primarily of a bodycentered tetragonal crystal structure phase which is stable with a sufficient amount of carbon dissolved therein in the solid state, or is unsusceptible to decomposition into a low temperature phase; and has for its object the provision of a permanent magnet material which has an excellent maximum energy product (BHmax), residual magnetic flux density (Br) and coercive force (BHc), and is also excelling in the other properties essential for a permanent magnet, such as magnetic characteristics, stability and mechanical properties. Namely, according to the present invention, there is provided a new permanent magnet material consisting primarily of a Mn-Al-C ternary alloy which has excellent magnetic characteristics, i.e. Bl'lmax 2 1.0 X 10 G.Oe., Br 2 2,800 G. and BHc 2 1,400 Oe., and excellent stability and mechanical properties.

Another object of this invention is to provide a permanent magnet material which is completely free from disintegration in the atmosphere caused upon hydrolysis of aluminum carbides, e.g. A1 C by the water present in air in the case where a ternary alloy constituting a magnet material contains an excessive amount of carbon therein, and which therefore has an excellent corrosion resistance.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram illustrating the compositions of the magnet alloys used in the example of the present invention;

FIGS. 2 to 4 inclusive are diagrams illustrating the relationship between the composition of the alloys and the magnetic characteristics thereof, in which FIG. 2 is a BI-lmax equivalent value curve, FIG. 3 is a Br equivalent value curve and FIG. 4 is 21 Elk equivalent value curve;

FIG. 5 is a diagram illustrating the compositions of the magnet alloys according to this invention; and

FIG. 6 is a diagram illustrating the relationship between the carbon amount in the magnet alloys and the Curie point of said alloys.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The Mn-Al-C alloys used in the experiment were selected from those whose compositions fall in the range definedby points A, B, C, D, E and F in FIG. I and were composed of 69 to 73 percent (by weight and so hereinafter) of Mn, 26 to 30 percent of Al and 0.5 to 2 percent of C. Within these ranges of the respective elements, a total of 62 sample alloys were prepared while varying the Mn and Al contents by 0.5 'percent and the C content by 0.2 percent from one sample to another. Namely, the sample alloys were prepared for each of the carbon contents consisting of 0.5, 0.7, 0.9, 1.1, 1.3, 1.5, 1.7 and 1.9 percent, and, when the carbon content was, e.g. 1.3 percent, the alloys prepared with this carbon content had the compositions of69 Mn 29.7 Al 1.3 C, 69.5 Mn29.2%A11.3%C,70%Mn28.7%A1 1.3% C, 70.5%Mn- 28.2%Al- 1.3 %C,71% Mn 27.7 Al -1.3%C,71.5%Mn27.2%Al-1.3%C,72%Mn 26.7%Al- 1.3%Cand72.5 %Mn26.2%A11.3%C.

These alloys may be cast by melting them in argon gas, carbon oxidegas, hydrogen gas or in the atmosphere and desired alloys may be obtained by melting the ingredients under the conditions suitable for the specific atmosphere used. In the example to be illustrated hereunder, the constitutional elements were molten mainly in an argon gas atmosphere. The melting points of these ternary alloys, of course, vary somewhat depending upon the compositions thereof but are about 1,260 C. (or in the range of l,240 to 1,280 C.).

In order to obtain Mn-Al-C ternary alloys having excellent magnetic characteristics, stability, mechanical properties and corrosion resistance, however, three elements of Mn, Al and C must be thoroughly alloyed and, for this purpose, itis necessary to establish the melting temperature within the range from 1,380" to 1,500 C.

Upon melting at the temperature specified above, Mn, A1 and C form a homegeneous melt, with carbon dissolved therein the solid state. Successively thereafter, the molten alloy is cast in a suitable mold at that temperature. In practice, the mold to be used must be suitably selected in accordance with the purpose, configuration and dimensions of the alloy desired, but the casting operating can be carried out relatively simply by the use of a die for the sake of experiment. In the production of a magnet adapted for use in power-generating lamps, motors and speakers, the shell mould casting, investment casting and sand mould casting methods are suitably employed. Upon completion of the casting, the alloys may be subjected to homogenizing and hot forging but such treatments are not always required.

Although the Mn-Al-C ternary alloys cast in the manner described show the characteristics required for permanent magnet as such, as will be described later, the magnetic characteristics may be improved greatly by subjecting them to a heat treatment in the manner described hereunder. Namely, the heat treatment basically consists of two steps, i.e. hardening and tempering (aging). The hardening is carried out by rapidly cooling the alloys from a temperature of 900 C. or higher in water, oil, air or an inert gas, irrespective of composition of said alloys, and the cooling rate must be 300 C./min. or higher. In this case, the temperature of the water, oil or air is only required to be 650 C. or below. In other words, the alloys must be cooled from 900 C. to 600 C. at the rate of at least 300 C./min. In general, the hardening is simply effected at the normaltemperature.

Upon completion of the hardening, the alloys are subjected to tempering successively thereafter. The conditions under which the tempering is carried out will largely affect the magnetic characteristics of the product alloys, and effective results can be obtained only under certain range of conditions. Namely, the tempering is effectively carried out for all compositions at a temperature of 480 to 650 C., and the period of tempering becomes longer as the temperature becomes lower and shorter as the temperature becomes higher. For instance, at a temperature of 600 C. a suitable period of tempering is about 80 minutes, and for some composition a period of 1.5 minutes was optimum at a tempering temperature of 650 C., while for some other composition a period of 24 hours was optimum at a tempering temperature of 550 C. In general, it appears that the characteristics of alloys containing a relatively large amount of Mn are improved with an elevating temperature and a prolonged period of tempering, and this relation is reversed for alloys containing a relatively small amount of Mn. However such relations have not been ascertained as yet. When alloys are hardened, for example, in a salt bathat 600 C. from a temperature of 900 C. or higher, tempering of the same may be accomplished successively at that temperature.

By the hardening and tempering as described above,,the cast Mn-Al-C ternary alloys are transformed from the hex agonal crystal structure of high temperature phase into a body-centered tetragonal crystal structure of stable magnetic phase having carbon forcibly dissolved therein in the solid state. It is to be noted, however, that the thus heat-treated alloys are not entirely transformed into the body-centered tetragonal crystal structure at every portions thereof but occasionally contain a very small amount of the other phases mixed therein which have no magnetism. These other phases include fiMn type solid solution, AlMn type compound phase, and some sorts of carbides of the hexagonal crystal structure of high temperature phase and of the phases described above with carbon'dissolved therein in the solid state or combined therewith.

The Mn-Al-C ternary alloys obtained in the manner described are much tougher than Mn-Al binary alloys and, for instance, the tensile strength, compressive strength and breaking strength of the former are about to times greater than those of the latter. The ternary alloys also have an improved workability providing for cutting work.

It is also to be noted that in the Mn-Al-C ternary alloys, the

magnetic phase (body-centered tetragonal crystal structure) thereof is highly stable to such an extent as to be considered as a stable phase, in contrast to that of the Mn- A1 binary alloys which is considered as a metastable phase, and such stable magnetic phase is hardly transformed into a low temperature phase. As stated above, the Mn-Al-C temaryalloys show the characteristics essential for a permanent magnet as they are cast and this is believed to be attributed to such a highly stable magnetic phase as mentioned above.

The magnetic characteristics of the Mn-Al-C ternary alloys, as they are cast, obviously vary depending upon the compositions thereof, but some of them shows a BHmax value of as high as about 0.7 to 0.8 M.G.Oe. However, these alloys as cast contain a considerable amount of high temperature phase remaining therein along with the magnetic phase. In contrast thereto, ithas been acknowledged that the alloys which had been subjected to the aforesaid heat treatment successively after casting, had substantially the entire phase thereof transformed into magnetic phase.

It has also been confirmed that the Mn-Al-C ternary alloys show better magnetic characteristics when they contain a very small amount of carbon-containing non-magnetic phase,

peculiar to the ternary alloys, in addition to the above-mentioned magnetic phase.

This indicates that the particularly excellent magnetic characteristics of the Mn-Al-C ternary alloys are attributed, not onlyto the aforementioned stable magnetic phase having carbon dissolved therein the solid state but also to the kind, amount and the state of distribution of the carbon-containing non-magnetic phase, peculiar to the Mn-Al-C ternary alloy system. However, the detail reasons are not clearly understood at the present stage because the non-magnetic phase is present in only an extremely small amount and distributed extremely finely. In any event, the non-magnetic phase is brought into existence when the amount of carboncontained in the alloys is 0.6 percent or more, and the alloys containing carbon in an amount of 0.6 percent or more are believed to be different in character from those which contain carbon in an amount of less than 0.6 percent. The fact that the charac teristics of alloys change at the border of carbon content of 0.6 percent, has also been confirmed by other experiment, e.g. by measuring the Curie point. The relationship between the carbon content and Curie point of alloys containing Mn and Al in the proportion of 7 2 to 28 is shown in FIG. 6. From this Figure, it is assumed that the maximum content of carbon in l the alloys with respect to Mn andAl is about 0.6 percent. It has also been acknowledged that the non-magnetic phase emerges in a large amount as a result of incomplete or no transformation of the structure into the body-centered tetragonal crystal structure or magnetic phase, when the tempering is carried out at temperatures outside the temperature range from 480 to 650 C.

The results of chemical analysis of the alloys formed in the manner described and the magnetic characteristics of said alloys are shown in Table 1 below.

TABLE 1 Sample 1 BHmax No. Mn Al C Br(g) BHc(Oe) (xl0' G.Oe.)

The magnetic characteristics shown in Table 1 were plotted into equivalent value curves as shown in FIGS. 2 to 4. The optimum conditions for the tempering, which is greatly influential on the magnetic characteristics of the product alloys, are variable depending upon the composition of the specific sample alloy but the equivalent value curves shown in FIGS. 2 to 4 were drawn using the values of the alloys which were tempered at 600 C. for 80 minutes, which are the temperature and the period to enable excellent characteristics to be obtained on all sample alloys.

As can be seen from FIG. 2, the value of Bl-Imax is greatest when the compositions of the alloys are approximating to 71.5 percent Mn 27.2 percent Al 1.3 percent C or 71.5 percent Mn 27 percent Al 1.5 C and, within the composition ranges of the alloys used in the experiment, the value of BI-Imax becomes equal to our greater than l.0 X G.Oe. when the amount of carbon is 0.6 percent or more and the amount of aluminum is 26.3 percent or more.

The value of Br, as is seen in FIG. 3, is highest with the composition in the vicinity of 69.5 percent Mn 29.5 percent Al 1 percent C but, with an amount of aluminum in excess to 26.4 percent, the value of Br becomes 2,800 G. or higher. Thus, the particularly excellent composition range for practical application with respect to Br value is extremely wide. Now, the value of BBC becomes 1,400 Oe. or higher when the aluminum content is 26.5 percent or more and carbon content is 0.6 percent or more, and reaches as high as 1,800 Oe. or even higher when the alloy composition is in the vicinity of 71.5 percent Mn 27 percent A] 1.5 percent C, as shown in FIG. 4.

From the overall results of the experiment described above and with particular reference to the BHmax which is most critical for magnet material, it may be said that alloys in the composition range of 69 to 73 percent Mn, 26.4 to 29.5 percent Al and 0.6 to 2 percent C show particularly excellent magnetic characteristics. This composition range is indicated in FIG. 5 by the area defined by points G, H, I, J and K. A further experiment conducted using additional samples has revealed the fact that the magnetic characteristics vary drastically between the carbon content of 0.5 percent and 0.6 percent, that the efi'ect of carbon is particularly noticeable when it is added in an amount of 0.6 percent or more, and that the magnetic characteristics vary greatly at the boundary of aluminum content of 26.4 percent.

The Mn-Al-C alloys in the composition range set out about have excellent magnetic characteristics for use as a permanent magnet but are not entirely satisfactory to be used for practical applications because of the following drawback. Namely, alloys containing carbon in an amount more than a certain limit form A1,, C;,, an aluminum carbide, which is hydrolyzed upon reaction with water present in air. This reaction is represented by the reaction formula provided below and the chemical analysis shows that this reaction is accompanied by the generation of methane gas. A1,, C 12 H 0 4M (OH) 3CI-I For this reason, the alloys are disintegrated into powder with the lapse of time after formation, making them unserviceable for practical uses. In the atmosphere, this spontaneous disintegration occurs in about one day after casting at the earliest and in several months after casting at the latest.

The periods before the commencement of disintegration in the atmosphere of the 62 sample alloys depicted in Table l are shown in Table 2 below for comparison. These periods were measured from the time of casting to the time when decomposition of A1 C was observed through a microscope. Those sample alloys which are not depicted in the Table below were not subjected to disintegration and therefore were proved to be excellent not only in moisture resistance and water proof but also in acid resistance and alkali resistance.

The line L L in FIG. 5 is a border line established one year after the production of the alloys and those alloys containing carbon in an amount of more than that represented by the line L L are susceptible to spontaneous disintegration. The disintegration tends to occur earlier as the carbon content moves away from the line L L. The observation through a microscope has revealed that the alloys which are susceptible to disintegration contain more amount and larger size of Al,C phase, whereas those which are less susceptible to disintegration contain only a trace amount of Al C; phase in their Mn- Al-C phases. Those sample alloys which were not disintegrated even one year after the casting contain no Al,C The border line is assumed to pass through the points representing the compositions of alloys of 69.5 percent Mn 29.5 percent AL 1 percent C, 71 percent Mn 27.5 percent A1 1.5 percent C and 72.5 percent Mn 25.5 percent Al 2 percent C, and in order to determine the position of said border line, the following experiment was conducted.

Namely, seven sample alloys were prepared which were composed of a fixed amount and 1.5 percent of C; 70.6 percent, 70.7 percent, 70.8 percent 70.9 percent, 71.0 percent 71.1 percent and 71.2 percent of Mn respectively; and the remainder of Al, and the periods before the commencement of disintegration of those sample alloys were measured, the result of which is shown in Table 3 below. In this case, the sample alloys containing 71.0 percent or more Mn were not disintegrated even after one year.

TABLE 3 Composition Period before disintegration 70.6 Mn 27.9 Al 1.5 C 6 months 70.7 Mn 27.8 Al 1.5 C 8 months 70.8 Mn 27.7 Al [.5 C l0 months 70.9 Mn 27.6 Al 1.5 C ll months Consequently, it may be concluded that, when the carbon content is located on or below the line L L' in FIG. 5, that is,

in the composition range defined by points M, l, J and N, those alloys are not susceptible to spontaneous disintegration.

The line L L can be obtained from the relationship between the amounts of C and Mn, as expressed by the formula,

C Mn 22.16 (percent), provided that the amount of Mn is in the range of 69 to 73 percent.

In the sample alloys mentioned above, the impurities in a total amount of up to 2 percent, which are usually contained in Mn, Al and C; will not particularly disadvantageously afiect the magnetic characteristics of the alloys.

The present invention which provides a magnet material which is composed of 69.5 to 73.0 percent Mn, 26.4 to 29.5

percent Al and not less than 0.6 percent but not more than (%Mn--22.16 percent of C and which has excellent magnetic characteristics, stability and mechanical properties as a permanent magnet and is excelling in corrosion resistance and unsusceptible to spontaneous disintegration in air, as described hereinabove, is a great industrial value.

What is claimed is:

1. A ternary magnet alloy composed of 69.5 to 73.0 percent by weight of manganese, 26.4 to 29.5 percent by weight of aluminum, not less than 0.6 percent but not more than (lMn 22.16 percent) by weight of carbon and essentially free of Alp I t I i UNITED STATES PATENT OFFICE CERTIFICATE CORRECTEQN Patent No. 3,661,567 Dated M y 1972 Inventor (s) Hiroshi Yamamoto ppears in the above-identified patent It is certified that error a hereby corrected as shown below;

and that said Letters Patent are Please insert the following missing claim for priority:

5653/64 of February 1, 1964 and Japanese No.

12679/64 of March 3, 1964-- Japanese No.

Signed and sealed this Z i-th day of April 1973.

(SEAL) Attest:

EDWARD M. FLETCHER, JR. Attesting Officer ROBERT GOTTSCHALK Commissioner of Patents USCOMM'DC 60376 FORM PO-105O (10-69) a u.s. covznumzm PRINTING orncz; I969 o-: 

